The invention relates to a circuit arrangement for compensating interference signals in the control loop of a linear lambda probe according to the features of the preamble of claim 1.
Legislators are using tax measures to promote the development of motor vehicles with lower fuel emissions and lower consumption of fuel.
In spark ignition engines with stoichiometric mixture formation (λ=1) this has led to the development of SULEV vehicles (Super Ultra Low Emission Vehicles) with extremely low emissions.
In order to save fuel, engines with direct high pressure injection of gasoline (HPDI, High Pressure Direct Injection) are currently being developed and introduced into the market. The fuel here is injected directly into the combustion space at increased pressure (approximately 150 bar). The conditioning of the mixture which is possible in this way can vary between rich, stoichiometric and lean. For the partial load operating mode of the engine a lean mixture formation offers considerable advantages in terms of consumption.
Both developments require a significantly more precise control of the mixture than is possible with currently customary lambda probes (binary step change probes). In addition, by binary step change probes have an extremely restricted measuring range around λ=1. They are therefore unsuitable for measurements in the lean operating mode λ>1.
For this reason, lambda probes with an extended linear measuring range, which are referred to as linear lambda probes, and circuit arrangements for operating them are being increasingly used.
FIG. 1 shows a linear lambda probe which is known per se. It has a heating element H, two electrode pairs VsC and IpC and a measuring chamber Mk which is connected to the exhaust gas stream A via a diffusion barrier GDP. The first electrode pair VsC is arranged between the measuring chamber Mk and air L and is used—similarly to the step jump probe—to measure the oxygen concentration in the measuring chamber Mk. The second electrode pair IpC is arranged between the measuring chamber Mk and the exhaust gas stream A. It permits—when a current Ip of appropriate polarity is applied—oxygen ions to be pumped out of the measuring chamber Mk or into it; hence the designation pump electrodes.
It is thus possible to generate a dynamic equilibrium between the flow of oxygen through the diffusion barrier and the flow of oxygen ions through the pair of pump electrodes. A suitable controlling criterion here is the oxygen concentration in the measuring chamber Mk which is determined using the measuring electrodes. A preferred value is, for example, Vs=450 mV for λ=1.
The pumping current Ip which flows in this case is a measure of the oxygen concentration in the exhaust gas. (And also of λ after numerical conversion).
Some lambda probes require an artificial oxygen reference for operation. This is produced by pumping oxygen out of the measuring chambers to the positive reference electrode Vs+ by means of a small current Icp (for example 25 μA). The oxygen concentration which is produced as a result is then used for its part as a reference point for measuring the oxygen concentration in the measuring chamber Mk. The evaluation circuit must make this current available.
The relationship between the oxygen concentration in the exhaust gas and the pumping current Ip is influenced by a number of probe parameters. For reasons of fabrication, the dynamic resistance of the diffusion barrier fluctuates. This would result in a deviation of the transmission ratio (gain errors). During fabrication, this is compensated by measuring and inserting a calibrating resistor Rc into the probe plug.
FIG. 2 shows a basic circuit diagram of a known device for operating a linear lambda probe of an internal combustion engine.
A first terminal Vs+, a second terminal Vp−/Vs−, a third terminal Vp+ and a fourth terminal Rc extend out of the probe S and are connected to the evaluation circuit. The probe heater and its terminals are not illustrated.
The inverting input of a controller is connected to the first terminal Vs+ of the probe S and its noninverting input is connected to a center voltage Vm (Vm≈Vcc/2) via a reference voltage Vref, Vcc (usually 5 V) being a supply voltage of the circuit.
The second probe terminal Vp−/Vs− and the inverting input of a pumping current source Ip Pump, whose noninverting input is connected to the output of the controller, are also connected to the center voltage Vm.
The output of the pumping current source Ip Pump is connected to the fourth input Rc of the probe S.
As the resistor Rc is subjected to considerable environmental loading owing to its installation position in the probe plug, a further resistor Rp is connected in parallel with it to the terminals Vp+ and Rc in the controller. This reduces the influence of a tolerance fault of Rc on the measuring accuracy of the pumping current Ip.
The method of operation of the known circuit arrangement illustrated in FIG. 2 for operating a linear lambda probe (without generating Icp) is as follows:
The terminal Vp−/Vs− of the probe is, like the reference voltage Vref, connected to the center voltage Vm. This serves as a reference voltage of the circuit.
The control amplifier R compares the Nerst voltage Vs of the probe with the reference voltage Vref (for example 450 mV) and generates an output voltage which is converted by the subsequent pumping current source I Pump into a corresponding current Ip which then flows through the pumping cell to the center voltage Vm. The pumping current brings about a change in the oxygen concentration in the measuring chamber of the probe, which in turns results in a change in the Nernst voltage Vs. The difference between Vs and Vref (=ΔVs) constitutes the control error of the loop. The pumping current Ip can be measured as a voltage drop at the resistor Rp/Rc. It is used as measure of the oxygen concentration in the exhaust gas.
In the stable control state (λ=1 in the measuring chamber), the Nernst voltage Vs is, for example, precisely 450 mV (ΔVs=0).
Equilibrium prevails between the oxygen flow through the diffusion barrier and the oxygen ion flow, caused by the pumping current Ip. The maximum range of the output voltage of the pumping current I Pump ranges from approximately 0.1 V to 4.9 V.
Alternatively, the control amplifier can also be embodied as an OTA (Operational Transconductance Amplifier) whose output stages form a current source. The output signal here is already a current and not—as is customary in the case of the operational amplifier—a voltage.
Furthermore, the dynamic resistance of the diffusion barrier has a temperature dependence and pressure dependence illustrated in FIGS. 3A and 3B, respectively which in turn causes faults in the transmission ratio. The temperature dependence is counteracted by measuring the probe temperature and controlling it by means of a heating element installed in the probe. For reasons of cost, a separate thermal element is not used here. Instead, the highly temperature-dependent internal resistance of the probe (probe impedance) is measured. The pressure dependence cannot be sensed in the probe by measuring equipment. If a separate pressure sensor is not used, an attempt is made to determine the dependence by means of a model-based calculation in the microcontroller and to compensate it numerically.
FIG. 4A shows the probe impedance Ris of the probe S and its temperature dependence. The probe impedance can be represented as a temperature-dependent, complex impedance with a plurality of RC elements as shown in FIG. 4B, in which case:    R1/C1 represents the contact resistance between electrodes and ceramic material,    R2/C2 represents the junction between the grain boundaries of the ceramic sintered elements, and    R3 represents the intrinsic resistance of the sintered material.
As one of the electrodes of the pumping cell is subjected to the exhaust gas, its internal resistance changes to a very great extent. For this reason, the Nernst cell Vs is used to measure the probe temperature. Here too, R1 changes and should therefore not be used for measuring temperature. As the time constant R1*C1 has the highest value (lowest frequency), it is possible to reduce its influence by suitable selection of the measuring frequency. The impedance of the series connection of R2/C2 and R1 is therefore measured. The impedance Ris of a typical linear lambda probe is approximately 100Ω at a temperature of approximately 500° C. to 700° C. (and a measuring frequency of 3 kHz).
Measurement of the internal resistance Ris:
A known measuring method for determining Ris is to apply an alternating current, for example 500 μA (peak-to-peak, abbreviated below to ss) to the probe terminal Vs+. As a result of this alternating current, an alternating voltage of 500 μA*100Ω=50 mV (ss) is produced at Ris and it is superimposed on the Nernst voltage Vs, the actual probe signal, as an interference signal.
FIG. 6 shows a typical voltage profile of the alternating voltage signal which forms an interference signal for the Nernst voltage. The signal is amplified in an amplifier V, for example by a factor 10, and then rectified in a rectifier GLR. The DC voltage Vri which has been produced in this way can then be fed to a microprocessor in order to control the temperature.
For example, the alternating current is generated, as illustrated in FIG. 5, by means of a 3 kHz square-wave oscillator OSZ which is supplied with a voltage Vcc. The signal is conducted to the probe terminal Vs+ via a high-impedance resistor R1 and a decoupling capacitor C1.
A basic problem of this circuit arrangement is the abovementioned mutual influence between Vs and this interference signal as this interference signal also appears at the input of the controller and constitutes a control error. The controller will attempt to compensate this control error within the scope of its bandwidth. To do this, it changes the pumping current Ip, which in turn has effects on the Nernst voltage Vs. As the pumping current Ip is the measured variable for λ, the primary probe signal is falsified. FIG. 7 makes this fact apparent. In the case of the 3 kHz signal (upper signal), the peak and the pulse tilt are falsified by the Vs signal, and in the case of the Vs Signal (lower signal) the 3 kHz triangular signal is undesired.